The present invention relates to a method of determining the carbon equivalent in structure modified cast iron, such as ductile and compacted graphite iron.
The binary phase diagram between iron and carbon is of limited interest in the foundry industry, because all materials which are used to produce cast iron always contain alloying elements such as silicon and manganese, together with impurities such as sulphur and phosphorous, which are able to change the phase relationships. Some of these elements can replace carbon in different proportions, and therewith influence the phase diagram. As a result of the total effect of the substances on the phase diagram, the liquidus temperature found at a specific composition of the melt, referred to as the "carbon equivalent" or C.E., can be expressed as EQU C.E.=% C+% Si/x+%P/y+. . .
where x is considered to assume values between 3 and 4 and y is considered to assume values between 3 and 6. In the U.S.A., this equation is normally simplified to EQU C.E.=%C+%Si/3
and this equation is accordingly used below.
This abbreviated formula can be used because the phosphorous content of those melts used within the foundry industry for treated cast iron is very low and therefore unimportant. The area of interest in the manufacture of compacted graphite iron and ductile iron fall within the range of C.E.=3 to 5%.
The majority of published iron-carbon-silicone phase diagrams relate to those conditions under which gray cast iron solidifies, i.e. an untreated iron in which the graphite crystals grow in an extended and branched flaky form. In this system, a eutectic reaction between .gamma. (austenite) iron and graphite flakes occurs at C.E. about 4.35% and at a temperature of about 1155.degree. C. Cast iron which has a carbon content or a C.E.&lt;4.35% is normally referred to as being hypo-eutectic, whereas materials which have a carbon content or a C.E. greater than 4.35% are referred to as being hyper-eutectic. As before mentioned, this definition is significant only with regard to flaky gray cast iron.
It is possible to determine the physical C.E. value of hypo-eutectic cast iron by means of the phase change temperature. A cooling curve will show a temperature arrest when the sample temperature passes the liquidus line and .gamma. phase begins to precipitate. The reason for this temperature arrest is because the growth kinetics of the austenite phase are very high and because the same also applies to the heat of crystallization of the .gamma. phase.
These factors contribute to form a sharp and well-defined point on the temperature-time-curve with the temperature arrest over a given period of time.
This principle has long been used in foundries. For instance, prior publication SE-B-350 124 teaches a device for establishing such a cooling curve for molten iron.
Attempts to use the same technique for the purpose of determining C.E. in hyper-eutectic alloys have not been successful, however. Flaky carbon is the first solid phase to precipitate from such a melt. The carbon crystals, however, will not nucleate immediately after passing the liquidus line, and the latent heat generated is insignificant and is spread over a temperature interval. Consequently, it is impossible to relate changes in the solidification curve to a well-defined phase conversion temperature which would enable C.E. to be determined.
This problem is solved by the method taught by SE-B-342 508. This publication discloses that when the formation of graphite can be suppressed by adding certain elements to the melt, the melt will be undercooled until the corresponding line in the metastable system, .gamma.-iron and cementite is reached. The first phase that is formed in highly hyper-eutectic melts during solidification will then be cementire, which, due to its high growth kinetics, will release sufficient heat to arrest the temperature decrease for a given period of time. The Applicant of the aforementioned patent publication SE-B-342 508 does not appear to be concerned about the fact that two completely different melts, the one hypo-eutectic with primary .gamma.-phase precipitation and the other hyper-eutectic precipitation of primarily cementite, will give the same result. The applicants in those prior art documents have also thus ignored the very important area of .gamma.liquidus displacement between the stable and metastable states. The applicant of the aforesaid patent publication SE-B-342508 also maintains that certain elements will suppress the formation of graphite and that tellurium, boron and cerium would appear to be the most effective elements, although magnesium is also mentioned in this context. Although this statement is partially true, millions of tonnes of ductile iron are produced annually with limited additions of cerium (and other rare earth metals and magnesium), with only a slight risk of cementite formation.